Editing Diffuse sky radiation (section)

==Color==
[[File:Trees-sky.jpg|thumb|upright=1.5|A clear daytime sky, looking toward the [[zenith]]]]

[[atmosphere of Earth|Earth's atmosphere]] scatters short-[[wavelength]] light more efficiently than that of longer wavelengths. Because its wavelengths are shorter,  blue light is more strongly scattered than the longer-wavelength lights, red or green. Hence, the result that when looking at the sky away from the direct incident [[sunlight]], the human eye perceives the sky to be blue.<ref>"[http://search.eb.com/eb/article-9062822 Rayleigh scattering]." ''Encyclopædia Britannica''. 2007. Encyclopædia Britannica Online. retrieved November 16, 2007.</ref> The color perceived is similar to that presented by a monochromatic blue (at wavelength {{nobr|474–476 [[Nanometer|nm]]}}) mixed with white light, that is, an [[Saturation (color theory)|unsaturated]] blue light.<ref>{{cite journal |author=Glenn S. Smith |url=http://www.patarnott.com/atms749/pdf/blueSkyHumanResponse.pdf |title=Human color vision and the unsaturated blue color of the daytime sky |journal=American Journal of Physics |volume=73 |issue=7 |pages=590–597 |date=July 2005 |doi=10.1119/1.1858479|bibcode = 2005AmJPh..73..590S }}</ref> The explanation of blue color by  [[John William Strutt, 3rd Baron Rayleigh|Lord Rayleigh]] in 1871 is a famous example of applying [[dimensional analysis]] to solving problems in physics.<ref>{{cite web|url=https://application.wiley-vch.de/books/sample/3527403205_c01.pdf|title=Craig F. Bohren, "Atmospheric Optics", Wiley-VCH Verlag GmbH, page 56.|website=wiley-vch.de|access-date=April 4, 2018}}</ref>

Scattering and absorption are major causes of the [[attenuation]] of sunlight radiation by the atmosphere. Scattering varies as a function of the ratio of [[particle size|particle diameters]] (of [[particulates]] in the atmosphere) to the wavelength of the incident radiation. When this ratio is less than about one-tenth, [[Rayleigh scattering]] occurs. (In this case, the scattering coefficient varies inversely with the fourth power of the wavelength. At larger ratios scattering varies in a more complex fashion, as described for spherical particles by the [[Mie scattering|Mie theory]].) The laws of [[geometric optics]] begin to apply at higher ratios.

Daily at any global venue experiencing [[sunrise]] or [[sunset]], most of the solar beam of visible sunlight arrives nearly [[tangent lines to circles|tangentially]] to Earth's surface. Here, the [[optical path|path]] of sunlight [[air mass (astronomy)|through the atmosphere]] is [[optical path length|elongated]] such that much of the blue or green light is scattered away from the line of perceivable visible light. This phenomenon leaves the Sun's rays, and the clouds they illuminate, abundantly orange-to-red in colors, which one sees when looking at a sunset or sunrise.

For the example of the Sun at [[zenith]], in broad daylight, the sky is blue due to Rayleigh scattering, which also involves the [[diatomic]] gases [[nitrogen|{{chem|N|2}}]] and [[oxygen|{{chem|O|2}}]]. Near sunset and especially during [[twilight]], [[Chappuis absorption|absorption]] by [[ozone]] ({{chem|O|3}}) significantly contributes to maintaining [[blue hour|blue color]] in the evening sky.